The present invention relates to a method and an apparatus for adjusting an extrusion die in an extruder.
An extruder for producing plastic profiles as are required for the production of plastic window frames for example are usually designed in such a way that an adapter part is provided adjacent to the extruder cylinder in which the endless screws are disposed, which adapter part forms the transition to the extrusion die. The cross-sectional progress of the respective profile is already substantially shaped in the extrusion die in order to be finally determined in the subsequent dry and/or wet calibration. In order to increase the productivity of the extrusion process, processing is carried out with ever increasing extrusion speeds. At the same time, demands placed on the quality of the produced profiles are also increasing. A high quality of the profiles can be achieved at higher extrusion speeds only with an extremely careful calibration of the extrusion dies with simultaneous adherence to a precisely defined operational state in the extruder. Such a calibration is only valid for a precisely defined raw material.
In order to detect the operational state of the extruder it is known to detect the pressure and the temperature of the melt in the region of the adapter part. For this purpose a pressure sensor and a temperature sensor are disposed in the zone of the flow channel of the melt for example. This allows detecting the operational state of the extruder. In the production of a new extrusion die, the calibration is performed by the manufacturer of the die. The procedure followed is such that the die is provided upstream with an extruder which is the same or similar to the extruder with which the die is operated after delivery by the customer. The relevant aspect for the calibration is also the use of the same basic material as in later production.
It has been noticed that at high extrusion speeds or during the extrusion of profiles with low wall thicknesses and generally under high requirements placed on the quality of the profile, such as in the case of narrow tolerances, only unsatisfactory results are achieved even in the case of optimal calibration of the die at the manufacturer after the delivery of the die to the customer. One reason is obviously that even extruders of the same design show slight differences which for example are caused by production tolerances, wear and tear or the like. The inventors of the present invention have recognized that such differences can be present even in cases where the measurements in the adapter part show identical values for temperature and pressure. In the inventors' opinion this is linked to the fact that the rheological properties of the plastic melt cannot be sufficiently described with the parameters pressure and temperature alone. One consequence of this fact is that after the assembly of the extrusion die at the customer's location, it is necessary to perform work-intensive calibration runs during which it is attempted to eliminate determined deficiencies in the profile by changing the extrusion conditions, such as the screw speed, heating output and the like. Since it is very difficult to find out which deviation is responsible for the occurrence of a certain deficiency, such as a deteriorated surface quality in a specific partial zone of the profile, such work is usually labor-intensive and cumbersome.
It is further known to flange so-called rheometer nozzles on an extruder, which nozzles are arranged as slotted nozzles. The material data thus obtained have proven to offer insufficiently meaningful information, especially in connection with the material of PVC, for the actual extrusion process, which means that a fine adjustment on the basis of the data thus gained is not possible.